A Photocycle for Hydrogen Production from Two-Electron Mixed-Valence Complexes

Esswein, Arthur J.; Veige, and Adam S.; Nocera, Daniel G.

doi:10.1021/ja054371x

Cited by 125 publications

(126 citation statements)

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“…The N-C-N angles in 2 and 3 were found to be 105.7(5) and 108.0(4)8 ( Figure 2 and Figure 3, as well as Table 1), slightly more acute than those found in II and III, reflecting the different N-C-N geometries induced by aryl versus bulky N-alkyl substituents. Because the N-C-N bond angle in 3 is greater than that in 2 and comparable to that of [ 2 , we conclude that the QBI core in 3 bears significant positive charge and thus has zwitterionic character, consistent with III. [83] Whereas the triazene linkers in 2 are nearly coplanar with the QBI core (N2-C1-N3-N4 17.3 (9)8), the isothiocyanate groups in 3 are nearly perpendicular (N1-C1-C5-S1 88.7 (6)8), that is, geometries similar to those observed in II and III (31.3(6) and 92.6(5)8, respectively).…”

Section: Introductionmentioning

confidence: 54%

“…From enantioselective synthesis [1] to the production of dihydrogen, [2] bimetallic catalysts have been employed in a variety of applications. Organometallic systems bearing N-heterocyclic carbene (NHC) ligands are useful catalysts for functional-group transformations and polymerization reactions, [3][4][5][6][7][8][9][10][11][12][13] and complexes featuring multitopic NHCs have recently emerged [14] as promising candidates for asymmetric and tandem catalysis, [15][16][17] as well as other applications.…”

Section: Introductionmentioning

confidence: 99%

“…[36][37][38][39][40] A synthetic bimetallic complex lacks the secondary coordination sphere that enzymes employ to encapsulate and stabilize metal-ligand geometries, and thereAbstract: Reaction of bromanil with N,N'-dimesitylformamidine followed by deprotonation with NaNA C H T U N G T R E N N U N G (SiMe 3 ) 2 afforded 1,1',3,3'-tetramesitylquinobis-A C H T U N G T R E N N U N G (imidazolylidene) (1), a bis(N-heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p-quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN 3 (2) 2 Cl] complexes 5 a and 5 b, respectively. Analysis of 2-5 by NMR spectroscopy and X-ray diffraction indicated that the electron-deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs.…”

Section: Introductionmentioning

confidence: 99%

“…We employed NHC coupling chemistry [62,63] to affix ferrocene (Fc) groups, [64] given that Fc displays welldefined electrochemical behavior. Additionally, we synthesized coordination complexes of 1 featuring [MA C H T U N G T R E N N U N G (cod)Cl] and [M(CO) 2 Cl] (M = Rh or Ir, cod = 1,5-cyclooctadiene) units. To explore how metal coordination affects quinobis(imid-A C H T U N G T R E N N U N G azolylidene) (QBI) electron density, the quinone stretching energies and reduction potentials in these complexes were analyzed by IR spectroscopy and electrochemistry.…”

Section: Introductionmentioning

confidence: 99%

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Quinobis(imidazolylidene): Synthesis and Study of an Electron‐Configurable Bis(N‐Heterocyclic Carbene) and Its Bimetallic Complexes

Tennyson

Ono

Hudnall

et al. 2009

Chemistry A European J

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Reaction of bromanil with N,N'-dimesitylformamidine followed by deprotonation with NaN(SiMe(3))(2) afforded 1,1',3,3'-tetramesitylquinobis(imidazolylidene) (1), a bis(N-heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p-quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN(3) (2) or FcNCS (3; Fc=ferrocenyl) or coordination to [M(cod)Cl] (4 a or 4 b, where M=Rh or Ir, respectively; cod=1,5-cyclooctadiene). Treatment of 4 a and 4 b with excess CO(g) afforded the corresponding [M(CO)(2)Cl] complexes 5 a and 5 b, respectively. Analysis of 2-5 by NMR spectroscopy and X-ray diffraction indicated that the electron-deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs. Infrared spectroscopy and cyclic voltammetry revealed that decreasing the electron density at ML(n) afforded an increase in the stretching energy and a decrease in the reduction potential of the quinone, indicative of metal-quinone electronic interaction. Differential pulse voltammetry and chronoamperometry of the metal-centered oxidations in 2-4 revealed two single, one-electron peaks. Thus, the metal atoms bound to 1 are oxidized at indistinguishable potentials and do not appear electronically coupled. However, the metal-quinone interaction was used to increase the electron density at coordinated metal atoms. Infrared spectroelectrochemistry revealed that the average nu(CO) values for 5 a and 5 b decreased by 14 and 15 cm(-1), respectively, upon reduction of the quinone embedded within 1. These shifts correspond to 10 and 12 cm(-1) decreases in the Tolman electronic parameter of this ditopic ligand.

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Section: Introductionmentioning

confidence: 54%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Quinobis(imidazolylidene): Synthesis and Study of an Electron‐Configurable Bis(N‐Heterocyclic Carbene) and Its Bimetallic Complexes

Tennyson

Ono

Hudnall

et al. 2009

Chemistry A European J

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“…In the presence of sacrificial chemical reductants, mononuclear and binuclear metal complexes of Co, Ni, and Rh are known to effect catalytic hydrogen evolution electrochemically or photochemically (28)(29)(30)(31)(32)(33)(34)(35)(36). Intimate mechanistic details, however, are known in only a few cases (37), and the different possibilities, such as protonation of a hydride vs. uni-or bimolecular reductive elimination (right side, WS1, Scheme 2), in general have not yet been unraveled.…”

Section: Methodsmentioning

confidence: 99%

Powering the planet: Chemical challenges in solar energy utilization

Lewis

Nocera

2006

Proc. Natl. Acad. Sci. U.S.A.

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7,368

5,540

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Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO 2 emissions in the atmosphere demands that holding atmospheric CO 2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Hydrogen Production from Water: Photocatalysis

Arachchige

Brewer

2005

Encyclopedia of Inorganic Chemistry

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Photocatalytic hydrogen production from water provides an attractive but challenging approach to meeting our global energy needs. Design and development of systems that utilize visible light to drive the energetically favorable, multielectron reduction of water to produce hydrogen fuel have been of long‐term interest. Addressing the challenges of efficient harvesting of solar energy to produce transportable fuels requires ongoing, sustained research in this field. Current technologies utilize photobiological methods, photoelectrochemical methods, and molecular photocatalysis. This article focuses on the recent progress of proton or water reduction using supramolecular photocatalysts. These photochemical molecular devices often couple charge‐transfer light absorbers (LAs) to reactive metals in complex molecular architectures. Linking multiple components into large molecular assemblies allows for applications requiring complex functions. Early successes in supramolecular photocatalysts for solar hydrogen production are summarized. The redox, excited state properties, and photochemical properties of supramolecular systems applicable in photocatalytic hydrogen production are presented. Fundamental studies of the perturbations of subunit properties upon incorporation into supramolecular arrays are paramount to their successful application in this forum. Optical excitation typically affords a metal‐to‐ligand charge‐transfer state that is quenched by electron transfer, providing reduced complexes with reactive metals able to deliver the electrons to substrates. Sacrificial electron donors provide coupling to oxidative chemistry, allowing independent study of photocatalysts and hydrogen production schemes. Supramolecular chemistry allows modulation of redox, photophysical, and photochemical properties by component modification. Coupling multiple LAs to an electron acceptor, which, when reduced by multiple electrons, provides photoinitiated electron collectors (ECs). Photoinitiated ECs, which collect multiple electrons on a reactive rhodium center with the supramolecular architecture remaining, reduce water to hydrogen fuel. Mixed‐valence systems that promote HX reduction to produce hydrogen have also been described. Coupling of charge‐transfer LAs to cobalt, iron, palladium, or platinum is a topic of recent interest in the design of water reduction photocatalysts. The general design considerations for the development of supramolecular assemblies that function as photocatalysts for proton reduction are discussed, providing valuable insight into engineering efficient solar hydrogen production catalysts for water reduction. Mechanistic studies remain important to understanding this complex multielectron photochemistry, which will aid in future molecular design. Much remains to be mastered in this complicated, multielectron photochemistry but recently there is increased awareness and interest in this field.

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A Photocycle for Hydrogen Production from Two-Electron Mixed-Valence Complexes

Cited by 125 publications

References 53 publications

Quinobis(imidazolylidene): Synthesis and Study of an Electron‐Configurable Bis(N‐Heterocyclic Carbene) and Its Bimetallic Complexes

Quinobis(imidazolylidene): Synthesis and Study of an Electron‐Configurable Bis(N‐Heterocyclic Carbene) and Its Bimetallic Complexes

Powering the planet: Chemical challenges in solar energy utilization

Hydrogen Production from Water: Photocatalysis

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